The basic objective of this study is to determine if stretch receptors in the bronchial wall play a role in the regulation of ventilation and to determine the site and mechanism of their function if they are found to play a role. Results of preliminary animal work done in this laboratory and previously reported clinical observations are consistent with the hypothesis that stretch receptors in medium-sized bronchi may be affected by changes in airway resistance and that these effects may in turn influence the regulation of ventilation significantly in health and disease. If such a hypothesis were proved correct, it could account for the known role of the work of breathing in regulating ventilation. More technically, the objectives of this study will be to determine the effects on blood gases and mechanics of breathing of obstruction of the airways of various sizes in the unanesthetized dog and from this drawing implications about the role of stretch receptors and their localization.